SHELL

Common shell scripting tips

Common shell scripting tips Shell scripting built-in commands Shell scripting special symbols

Wednesday, September 22, 2021 Read

How to use a here documents to write data to a file in bash script

How to use a here documents to write data to a file in bash script allow here-documents within shell scripts to be indented in a natural fashion using EOF<<- command <<-EOF msg1 msg2 $var on line EOF Disabling pathname/parameter/variable expansion, command substitution, arithmetic expansion with 'EOF' #!/bin/bash OUT=/tmp/output.txt echo "Starting my script..." echo "Doing something..." # No parameter and variable expansion, command substitution, arithmetic expansion, or pathname expansion is performed on word. # If any part of word is quoted, the delimiter is the result of quote removal on word, and the lines in the here-document # are not expanded. So EOF is quoted as follows cat <<'EOF' >$OUT Status of backup as on $(date) Backing up files $HOME and /etc/ EOF echo "Starting backup using rsync..."

Friday, May 7, 2021 Read

How to Create Temporary Files in Bash: mktemp and trap

How to Create Temporary Files in Bash: mktemp and trap mktemp command The generated temp file name is random and readable/writable only by the user. To ensure creation succeeds, append OR (||) after mktemp to exit on failure. To delete temp files on script exit, use trap to register cleanup. #!/bin/bash trap 'rm -f "$TMPFILE"' EXIT TMPFILE=$(mktemp) || exit 1 echo "Our temp file is $TMPFILE" mktemp options -d creates a temporary directory. -p specifies the directory for temp files. Default is $TMPDIR. If not set, /tmp is used. -t specifies a filename template. The template must end with at least three consecutive X characters for randomness; six X are recommended. The default template is tmp. followed by ten random characters. trap usage The trap command handles system signals in Bash scripts.

Monday, December 30, 2019 Read

Shell Script Study Notes

Shell Script Study Notes Arithmetic operations val=`expr $a + $b` Operators Symbol Description Example ! NOT [ ! false ] -o OR [ $a -lt 20 -o $b -gt 20 ] -a AND [ $a -lt 20 -a $b -gt 20 ] = equality check [ $a = $b ] != inequality check [ $a != $b ] -z string length is 0, returns true if 0 [ -z $a ] -n string length is not 0, returns true if not 0 [ -n $a ] str check whether string is empty, true if not [ $a ] -b check whether file is a block device [ -b $file ] -c check whether file is a character device .. -d check whether file is a directory [ -d $file ] -f check whether file is a regular file [ -f $file ] -r check whether file is readable .. -w check whether file is writable .. -x check whether file is executable .. -s check whether file is empty .. -e check whether file exists .. Special variables Variable Meaning $0 file name of the current script $n arguments passed to a script or function; n is the position $# number of arguments passed to a script or function $* all arguments as a single word, e.g., “1 2 3” $@ all arguments as separate words, e.g., “1” “2” “3” $? exit status of the last command or return value of a function $$ current shell process ID; for scripts, the PID of the script process POSIX program exit statuses Code Meaning 0 command exited successfully > 0 failure during redirection or word expansion (~, variables, commands, arithmetic, and word splitting) 1 - 125 command exited unsuccessfully; specific meanings are command-defined 126 command found but file is not executable 127 command not found > 128 command died from a signal Input/output redirection Command Description command > file redirect output to file command > file redirect output to file by appending n > file redirect file descriptor n to file n » file redirect file descriptor n to file by appending n >& m merge output file m and n n <& m merge input file m and n « tag use content between start tag and end tag as input File include Use . or source to include files.

Wednesday, August 7, 2019 Read

What does `< <(command args)` mean in the shell?

What does < <(command args) mean in the shell? while IFS= read -r -d $'\0' file; do dosomethingwith "$file" # do something with each file done < <(find /bar -name *foo* -print0) <() is called process substitution in the manual, and is similar to a pipe but passes an argument of the form /dev/fd/63 instead of using stdin. < reads the input from a file named on command line. Together, these two operators function exactly like a pipe, so it could be rewritten as

Saturday, November 17, 2018 Read