Unix shell basics

Rough notes for a half-hour course on Unix shell basics.

Run a program (ls)

• ls
• which ls
• /bin/ls
• cd /bin
• ./ls
• echo $PATH
• cd -

Shell variables

• echo PATH
• echo $PATH
• PATH=~
• echo $PATH
• ls (!)
• put PATH back to original value

Manuals

• man ls
• man uses an external PAGER program, by default "less"
  • less keys (similar to vi):
    • h help
    • q quit
    • SPACE scroll forward
    • / search
    • n next search hit
  • less ../stb-tester/stbt.py
  • cat ../stb-tester/stbt.py
  • PAGER=cat man ls
• shell builtins: man cd
  • also: help cd
• man man
  • man passwd
  • man 5 passwd
  • man printf
  • man 3 printf
  • "ls(1)" means the ls manpage in section 1
• man hier for a somewhat old-fashioned description of the file-system hierarchy

arguments

• cat printargs.c

    #include <stdio.h>
  
    int main(int argc, char** argv) {
        int i;
        for (i = 0; i < argc; ++i) {
            printf("--- arg %d: ---\n", i);
            printf("%s\n", argv[i]);
        }
        return 0;
    }
  

• gcc printargs.c -o printargs
• ./printargs hello world

expansion

• Shell Parameter Expansion
  • http://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html
  • ./printargs $PATH
• Single Quotes
  • http://www.gnu.org/software/bash/manual/html_node/Single-Quotes.html
  • ./printargs 'hello world'
  • ./printargs '$PATH'
• Double Quotes
  • http://www.gnu.org/software/bash/manual/html_node/Double-Quotes.html
  • ./printargs "$PATH"
  • ./printargs "$PATH hello world"
  • ./printargs $PATH hello world
• Does word splitting happen before or after parameter expansion?
  • DAVE="a b c"
  • ./printargs $DAVE
  • ./printargs "$DAVE"
• Filename Expansion
  • http://www.gnu.org/software/bash/manual/html_node/Filename-Expansion.html
  • ./printargs *.c
  • ./printargs *
  • ls *
• Other Shell Expansions
  • http://www.gnu.org/software/bash/manual/html_node/Shell-Expansions.html
  • ./printargs ~

environment variables

• cat printenv.c

    #include <unistd.h>
    #include <stdio.h>
  
    extern char** environ;
  
    int main(int argc, char** argv) {
        char** env;
        for (env = environ; *env; ++env) {
            printf("%s\n", *env);
        }
        return 0;
    }
  

• gcc printenv.c -o printenv
• ./printenv
• STBT_CONFIG_FILE=~/.config/stbt/stbt.conf
• ./printenv
• STBT_CONFIG_FILE=~/.config/stbt/stbt.conf ./printenv
• ./printenv
• export STBT_CONFIG_FILE=~/.config/stbt/stbt.conf
• ./printenv
• env - A=1 B=2 ./printenv

standard input, output & error

• Standard Input is where input data comes from, often the keyboard.
• Standard Output is where output data goes, often the terminal screen.
• grep o and type "hello" (RETURN) and "there" (RETURN) (on standard input)
• Redirections
  • http://www.gnu.org/software/bash/manual/html_node/Redirections.html
  • grep main < printenv.c Redirect input
  • grep main < printenv.c > main.txt Redirect output
• Standard error
  • grep < printenv.c > main.txt You wouldn't want the error going into main.txt
  • Not just for errors:
    • curl http://www.google.com
    • curl http://www.google.com > google.html
• Pipelines
  • http://www.gnu.org/software/bash/manual/html_node/Pipelines.html
  • wc -l and type a few lines, then Control-D on an empty line
  • cat printenv.c | wc -l
  • wc -l < printenv.c
  • cat printenv.c | grep int | wc -l

job control

• http://www.gnu.org/software/bash/manual/html_node/Job-Control.html
• sleep 600 press Control-C
• sleep 600 press Control-Z
• jobs Stopped
• bg %1
• jobs Running
• man bash page down a bit, then press Control-Z
• jobs
• fg then press Control-Z again
• sleep 700 &
• jobs
• kill %3 Same as sending Control-C to a foreground process
• ps f
• kill 1234 Give the process id of the sleep process instead of "1234"

exit code

• Just an arbitrary number to communicate exit status to the calling process
• ./printargs
• echo $?
  • See http://www.gnu.org/software/bash/manual/html_node/Special-Parameters.html
• Modify printargs's main function to return 42
• gcc printargs.c -o printargs
• ./printargs
• echo $?
• The shell considers a 0 exit code as success, anything else as failure.
• Also used by shell looping & conditional constructs:
  • if ./printargs; then echo ok; else echo failed; fi
  • ./printargs && echo ok Short-circuiting boolean AND
• man grep Scroll down to EXIT STATUS near the end

Further reading

• I recommend reading the original manuals instead of unreliable googled tutorials.
• intro(1) i.e. type: man 1 intro
• bash(1) -- but use it as a reference; don't attempt to read it cover-to-cover
  • http://www.gnu.org/software/bash/manual/
  • Do read sections "Shell Commands", "Shell Expansions", "Redirections", and "Builtin Commands".
• GNU coreutils categorised index: http://www.gnu.org/software/coreutils/manual/coreutils.html
  • and in particular the last chapter, "Opening the Software Toolbox": http://www.gnu.org/software/coreutils/manual/coreutils.html#Opening-the-software-toolbox
• Not all Unix (or even Linux) systems use the GNU user-space tools
  • Busybox is common in the embedded-Linux world: http://busybox.net/downloads/BusyBox.html
• Fedora System Administrator's Guide (if you're using Fedora): http://docs.fedoraproject.org/en-US/Fedora/17/html/System_Administrators_Guide/